Nowadays presensitized plates (i.e., PS plates) using, e.g., a positive-type photosensitive material which contains a diazo compound and a phenol resin as main components and a negative-type photosensitive material which contains an acrylic monomer or prepolymer as a main component, are used in practice as lithographic offset printing plates. These plates are all low in sensitivity, and so in the exposure operation for producing therefrom the printing plates it is required of them to be in close contact with an original film on which images are previously recorded. On the other hand, owing to progress in both computer technology, including graphic processing and bulk data storage, and data communication technology, there has lately been put to practical use an electronic editing system in which a series of operations, involving input of originals, amendment, compilation, layout and page make-up, are performed from first to last with a computer and the thus edited originals are transmitted immediately as the output to remote terminal plotters by a high-speed communication network or communications satellite. In particular, there is a great demand for the electronic editing system in the field of newspaper printing which requires the immediacy. Further, in a field such that original manuscripts are stored in the form of film and printing plates are reproduced from the films picked out among the stored ones in answer to requests, it can be expected that the development of bulk recording media such as an optical disc enables those originals to be stored as digital data in such recording media.
However, there are few, if any, practically usable direct type printing plates, or printing plates produced directly from the output of a terminal plotter. Even in the case that the electronic editing system is working, therefore, it is the present situation that a printing plate is produced by the method comprising the steps of recording the output on a silver salt photographic film, bringing the resulting film into contact with a presensitized plate (PS plate) and then performing an exposure operation. One reason for adoption of this method is that there have been difficulties in developing presensitized plates which have sensitivities sufficient for the production of direct type printing plates within a practical time by the use of the light source of an output plotter (e.g., He--Ne laser, semiconductor laser).
As a photosensitive material having high photosensitivity enough to provide a direct type printing plate, an electrophotographic photoreceptor can be thought of. Specific examples of an electrophotographic photoreceptor include those disclosed in JP-B-37-17162 (the term "JP-B" as used herein means an "examined Japanese patent publication"), JP-B-38-6961, JP-B-38-7758, JP-B-41-2426, JP-B-46-39405, JP-A-50-19509 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-50-19510, JP-A-52-2437, JP-A-54-145538, JP-A-54-134632, JP-A-55-105254, JP-A-55-153948, JP-A-55-161250, JP-A-57-147656 and JP-A-57-161863.
As a method of producing a printing plate by the use of electrophotography, there is already known the method in which the non-image area of a photoconductive layer is removed after the toner-image formation.
In the foregoing method, a binder resin of the kind which can be eliminated through dissolution or swelling in an alkaline solvent is used as the binder resin of an electrophotographic photoreceptor, and the area free from toner images is eluted using the toner images as resist to result in exposure of the water receptive face, thereby producing a lithographic printing plate.
However, printed matter obtained with the thus produced lithographic printing plate sometimes suffers scumming, namely gets ink on the non-image area. The scumming is presumed to arise from incomplete removal of the photosensitive layer from the water receptive substrate by elution. The lithographic printing plate using an electrophotographic photoreceptor stored for long time has more serious scumming problem. As a cause of the aggravation of scumming, it can be thought that a binding resin used in the photoreceptor comes to have an increased adhesiveness to the water receptive substrate (conductive support) by the influence of water or the like to result in more incomplete removal of the photoreceptive layer in the non-image area from the water receptive substrate in the elution process. The scumming can be prevented by increasing the solubility of the elute. Therein, however, the elution proceeds also in the horizontal direction of the image area (side etching) to deteriorate the image quality.
With the intention of solving the above problem, it has been tried to increase the acid content in a binder resin. While increase of the acid content in a binder resin can bring about an improvement in eluting properties, it involves problems from a practical point of view. That is, it not only produces an adverse effect on the scumming because the adhesiveness to a water receptive substrate is built up through long-term storage but also deteriorates electrophotographic characteristics, particularly charge accepting and charge-retaining characteristics.